Evaluation of Ru-Ti Electrode-Based TSM Langasite Resonators for High-Temperature Applications

High-temperature (HT) properties of a thickness-shear mode (TSM) langasite resonator with Ru-Ti electrodes are reported for the first time. Resonators with 300 nm Ru and 15 nm Ti films as the primary and adhesive electrode layers, respectively, were investigated and compared against those with Au-Cr and Au-Ti electrodes. HT stability of the fabricated samples under continuous excitation were examined up to 750 °C by monitoring their morphological changes, sheet resistance, resonance parameters, and their equivalent circuit elements. Results indicate that for Ru-Ti electrodes, a polycrystalline RuO₂ cover layer was formed on the surface of Ru, which protected the underlying layer from further oxidation. Consequently, the electrical and motional resistances of the Ru-Ti sample experienced the least change post-annealing, which was also reflected in its ability to retain the highest Q -factor after heat treatment. Ru-Ti-based resonator also exhibited comparable performance to other samples in terms of resonant frequency shifts and second-order temperature coefficients, further strengthening the position of Ru as a suitable alternative to other electrode materials.